Bone growth is desirable in many instances, such as when vertebrae in a patient's spine are fused to overcome pain and other effects caused by inter-vertebral movement or intra-vertebral movement. Although bone growth occurs naturally, it can be stunted or stopped by various factors such as tobacco, alcohol and steroid usage, poor bone stock, and age. Moreover, stimulating bone growth to speed recovery is desirable in some instances such as when an injured athlete wishes to return to her sport quickly. Thus, there is a need for stimulating bone growth in individuals.
Bone growth can be stimulated by various means. One such means for stimulating bone growth is by passing an electrical current through the bone. When fusing vertebrae in a patient's spine, various means have been used to stimulate bone growth. For example, some stimulators include wire electrodes embedded in bone fragments grafted to a region of the patient's back containing the vertebrae to be fused. Direct electrical current is applied to the electrodes to stimulate bone growth and fuse the fragments and adjoining vertebrae. To permit the current to be applied for extended periods of time while permitting the patient to be mobile, a generator is connected to the wire electrodes and implanted between the skin and muscle near the patient's vertebral column. The generator provides a continuous low amperage direct current (e.g., 20 μA) for an extended period of time (e.g., six months). After the vertebrae are fused, the generator and leads are surgically removed. Although these embedded electrodes are generally effective, the wire electrodes are susceptible to failure, requiring additional surgery to repair them. Moreover, placement of the wire electrodes is less than precise, allowing some of the current to pass through areas of tissue and bone where it is unneeded and where the current could potentially have adverse effects. Further, imprecise placement may require more energy to be provided to the electrodes than otherwise necessary to be optimally effective. Thus, there are several drawbacks and potential problems associated with devices such as these.
Although small amounts of bone movement can stimulate growth, it is generally desirable to limit movement between the bones or bone fragments being fused. There are several known means for limiting bone movement. Among these means for limiting bone movement are plates, rods and screws.
The plates and rods are typically held in position by screws which are mounted in the bone or bones being fused. FIG. 1 illustrates screws (generally designated by 10) driven into a vertebra 12 to immobilize the vertebra. As previously mentioned, the screws 10 are used for attaching rods 14 and/or plates (not shown) to vertebrae to hold the vertebrae in position while they fuse. Although these screws work well for their intended purpose, they do not facilitate electrically stimulating the region. Moreover, if electrical stimulation were applied to bones having conventional screws, the screws could potentially conduct current to areas of tissue and bone where the current is unneeded and where the current could potentially have adverse effects. Thus, there are drawbacks and potential problems associated with conventional screws such as these.
Beyond the well defined role of electrical fields within bone formation, electrical fields have also shown significant promise in aiding healing and recovery in nerve and spinal cord injury. Stimulating tissue healing with electrical currents has been demonstrated to be efficacious in animal models and is now being attempted experimentally in human subjects. Further, spinal cord and nerve root injury has been known to cause associated debilitating pain syndromes which are resist treatment. These pain syndromes also have shown improvement with pulsed electrical stimulation. Given these findings it is envisioned that apparatus providing a specified and confined electrical field through bony constructs and adjacent tissue (e.g., neural tissue) will facilitate an enhanced recovery from spinal cord and nerve injury, including improved functional outcome, better wound healing, and a higher level of pain control.